Aaron Carroll and Rachel Vreeman debunk sex myths in their new book. / St. Martin's Griffin

by Liz Szabo, USA TODAY

by Liz Szabo, USA TODAY

Of the many health myths in circulation, those about sex are among the most difficult to eradicate, partly because they're spread on the playground, say authors of a new book that aims to clear up some of the most common misconceptions.

"A lot of what we know about sex comes from really terrible sources," says Rachel Vreeman, co-author with Aaron Carroll of Don't Put That in There! And 69 Other Sex Myths Debunked (St. Martin's Griffin, $13.99).

"Even as adults, we are still often relying on what we heard in the fourth grade," says Vreeman, an assistant professor of pediatrics at Indiana University's School of Medicine. "Misinformation about sex sticks with us longer than other misinformation, maybe because we are too embarrassed to ask about it."

After dispelling so many myths, is there really enough misinformation to fill another book?

When it comes to sex, yes.

"That's what everyone wants to hear about," Carroll says.

Carroll's online video on the new book received more than 2.4 million views on YouTube, compared with the 30,000 or so for his analysis on the health care system in Germany.

"More people will watch that video on '20 Sex Myths' than everything else I will ever do in my career combined," says Carroll, director of Indiana University's Center for Health Policy and Professionalism Research.

One of the biggest misconceptions about sex is that people are having lots of it. Vreeman notes that 57% of people ages 24 to 30 had no sex in the past year. "You're not alone if you haven't had sex," she says. "You're not a freakish minority."

People also falsely assume that kids are having more sex than ever, partly because of more sex in pop culture, Carroll says.

In 1988, about half of boys ages 15 to 17 were having sex. By 2010, that rate had been cut in half, Carroll says. Among girls the same age, rates of sexual activity fell from 37% to 27%.

Carroll notes that the teen birth rate has been declining for two decades and is now at a historic low. And in spite of the widespread notion that kids are awash in pornography, Carroll says studies show that only 40% of teen boys - often considered a prime audience for sex videos - viewed porn recently. Though that number may be higher than parents would like, Carroll says, it suggests that boys are actually less sex-obsessed than often portrayed.

"At the same time that we feel like everything is so sexualized, kids are actually having a lot less sex," Carroll says. "People think it's the worst it's ever been, but teen pregnancy is at its lowest rate in decades."

And while Vreeman says she tends to "joke like a sixth-grade boy" when talking about sex, she says she does have an interest in helping educate young people about health.

"There are some basic educational ideas that I want young adolescents to know and understand, that these things are healthy and there is nothing wrong with you if you are thinking about this," Vreeman says. "And as a specialist in HIV-related care, I do want people to know how to protect themselves."

For example, Vreeman and Carroll cite studies that disprove the claim that girls become more promiscuous if they're vaccinated against HPV, a family of viruses that causes cervical cancer. And the authors say they felt compelled to debunk the myth that women can't become pregnant as the result of a rape - a statement that landed former Missouri congressman Todd Akin in hot water in 2012 and may have cost him the election.

Other myths include: Athletes need to abstain before the "big game" to perform well on the field; the size of a man's feet is proportional to the size of other organs; sex can cause heart attacks; and sex burns lots of calories. All are false, authors write.

Though people can burn up to 150 calories during a half-hour of sex, for example, the authors note that very few people actually get that much exercise. In fact, sex for most people lasts only five minutes. And even in a best-case scenario, in which people burn 100 calories per round of lovemaking, it would take 35 such sessions to lose a single pound.

Yet even the authors say they've been taken in by myths. Carroll says he believed the one about women peaking sexually in their 30s, and Vreeman fell for the claim that wearing a bra prevents breasts from sagging. There's no evidence to support either idea, they say.

"You start to wonder, 'Do I know anything?' " Carroll says. "You really have to go back to the original studies. It's just too easy to assume what you believe is true."